Two-stage flotation purification of NaOH

ABSTRACT

A process for purifying an aqueous solution of NaOH by removing suspended NaCl and Na 2  SO 4  impurities therefrom comprising bubbling a gas into the impurity-containing NaOH solution at a first stage temperature above 16° C. up to 31° C. and at a second stage temperature above 31° C. up to about 36° C., separating a foam layer containing NaCl and Na 2  SO 4  from NaOH at the first stage and a foam layer containing Na 2  SO 4  from NaOH at the second stage.

BACKGROUND OF THE INVENTION

This invention concerns a two-stage flotation purification system for removing suspended NaCl and Na₂ SO₄ impurities from aqueous NaOH having a concentration of less than 52 percent by weight of NaOH. The process is well suited to purification of caustic soda manufactured by electrolysis of brine in cells utilizing diaphragm partitions.

It is known to remove suspended NaCl and Na₂ SO₄ impurities from aqueous sodium hydroxide by filtration and by centrifugation. Filtration purification is a costly, difficult to operate procedure. Centrifugation is difficult to control because it is so sensitive to crystal size.

Purification of aqueous sodium hydroxide by flotation of NaCl has been disclosed in Japanese patent application No. 52-69895. However, that purification relies on use of organic surface active agents and does not contemplate the two-stage, temperature-related separation described herein. Use of organic surface active agents is undesirable because of the difficulty of removing them from recycled NaCl and from NaOH product. The surface active agents are impurities in the NaOH product. Also, they may damage cell anodes when returned with recycled NaCl to the electrolysis cells.

U.S. Pat. No. 4,065,270 discloses the flotation separation of crystals of impurities from a slurry of sodium hydroxide hydrate crystals. The patent does not contemplate separation of impurity crystals from aqueous sodium hydroxide solution nor does it contemplate the two-stage, temperature-related separation described herein. Production of sodium hydroxide hydrate crystals as taught by the patent is avoided in the process of this invention wherein such crystals would become entrained with those of the very impurities from which NaOH is desired to be separated.

SUMMARY OF THE INVENTION

This invention concerns a process for purifying an aqueous solution of less than 52 weight percent of NaOH by removing suspended NaCl and Na₂ SO₄ impurities therefrom. The process comprises:

(i) bubbling a gas into the impurity-containing NaOH solution at a temperature above 16° C., up to and including 31° C., said gas transporting the suspended impurities to the surface of the NaOH solution, the suspended impurities forming a surface foam layer on the NaOH solution;

(ii) separating the surface foam layer of impurities from the NaOH solution;

(iii) bubbling a gas into the NaOH solution from step (ii) at a temperature above 31° up to about 36° C., said gas transporting the suspended impurities to the surface of the NaOH solution, the suspended impurities forming a surface foam layer on the NaOH solution;

(iv) separating the surface foam layer of impurities from the aqueous NaOH solution;

whereby substantially all of the suspended NaCl impurity is removed by steps (i) and (ii);

whereby substantially all of the suspended Na₂ SO₄ impurity remaining after steps (i) and (ii) is removed by steps (iii) and (iv).

Preferred process conditions comprise an aqueous NaOH feed of from 45% to <52% NaOH; a first stage temperature of 25° C. to 29° C.; and a second stage temperature of 32° C. to 34° C.

The "gas" employed to float the impurities to the surface can be any gas inert to the contents of the impurity-containing NaOH solution which it will contact. Preferred gases are air and nitrogen; most preferred is air.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow diagram of one representative two-stage flotation purification system of this invention.

FIG. 2 is a schematic flow diagram of a second representative two-stage flotation purification system of this invention. This Figure, for the sake of simplicity, does not show gas introduction means.

There are several alternative systems to accomplish the flotation purification depicted in FIGS. 1 and 2. The alternatives will be readily apparent to one skilled in the art from the description provided herein. All of said systems are contemplated to fall within the scope of this invention. Alternatives include multistage processing at successively increasing temperatures; heat supplied in the successive vessels or stages; paddle skimmers or spillover decantation for foam impurities; turbulent gas entry, and the like.

Although FIGS. 1 and 2 do not refer to foam-entrained NaOH for the sake of simplicity, it will be appreciated that the surface foam layer will contain some aqueous NaOH, perhaps as much as 35% or more by weight. In any event, the surface foam layer will always be richer in impurities and leaner in NaOH than the underlying aqueous NaOH layer.

DETAILS OF THE INVENTION

There are several factors which affect purification of aqueous NaOH. For example, there is an NaOH concentration/temperature relationship. The concentration/temperature relationship makes it necessary that the aqueous NaOH feed be less than 52% by weight of NaOH. More concentrated than 52% and NaOH hydrate crystals might well form at the temperatures at which this process is designed to be run. The presence of NaOH hydrate crystals would lead to their entrainment with the NaCl and Na₂ SO₄ impurities. Thus, the efficiency of the process would be seriously compromised.

Another important factor in appreciating the scope of this invention is that the disclosed process is most beneficially employed in a continuous process. In a batch process, time can be allocated to allow impurity crystals to settle. The supernatent aqueous NaOH would then be correspondingly free of the impurity. However, even if it were convenient to employ such a batch process with the relatively unproductive time required for settling impurities, there would remain the problem of recovering and discarding the impurity sediment.

The disclosed process although perfectly acceptable in a batch operation (even one in which some settling of impurities takes place) is more efficiently utilized in a high throughput continuous operation. It is economically more attractive to collect foam impurity continuously from the top of a relatively small flotation vessel than to utilize relatively larger settling tanks from which sedimental impurities must be continuously purged to avoid hardcake formation with attendant removal problems.

The process of this invention will allow purification of aqueous NaOH to the degree that substantially all suspended impurities are removed. If desirable, however, process times can be controlled, as will be obvious to those skilled in the art, so that less than all suspended impurities are removed. In any event, the process of this invention will readily lead to production of caustic having 1.10% or less of NaCl and 200 ppm or less of Na₂ SO₄, approximately all of which is in solution.

Although not wishing to be bound by this hypothesis, it is believed that the process of this invention may be dependent on two factors: solubility of the impurities and viscosity of the feed. At first stage temperatures, solubility and viscosity conditions are such that substantially all the suspended NaCl is transported to the surface by the gas which is introduced. However, such temperatures are lower than ideal for the substantially complete removal of Na₂ SO₄.

Effective second stage removal of substantially all remaining suspended Na₂ SO₄ is predicated on the lower viscosity of the feed at second stage temperatures. The lower viscosity allows the gas to transport the remaining Na₂ SO₄ to the surface under normal commercially practical operating conditions. Concerning viscosity, in the case of 50% caustic, the viscosity versus temperature curve is steep: viscosity at 35° C. being 28 cp and at 27° C. being 44 cp, a 36% difference.

The rate at which air or other gas is fed to the first and second stages is dependent on the feed rate of the caustic. As will be obvious to one skilled in the art, the gas and caustic feed rates can be adjusted to attain relatively splash-free flotation with minimum NaOH entrainment. Gas and caustic feed rates can also be readily adjusted with a view to maintaining a certain level or depth of foam, say, per unit time or per pass of the skimmer paddle, etc. In any event, one skilled in the art, based on this disclosure, will have no difficulty in emplacing the disclosed purification process in any existing NaOH production process.

The following purification procedures were employed to remove NaCl (in weight percent) and Na₂ SO₄ (in ppm by weight) in the first stage (F.S.) of the two-stage flotation purification system of this invention.

FIRST STAGE SEPARATIONS Nos. 1 and 2

Two identical 400 ml samples of aqueous NaOH were treated at room temperature as follows: The first sample was transferred to a 600 ml beaker and sparged with nitrogen for 3 hours at a moderate sparging rate. After sparging, the sample was allowed to remain stationary for 20 minutes before extracting 100 ml of sample from below the accumulated foam layer on the top.

The second sample was also transferred to a 600 ml beaker and sparged for 1 hour with nitrogen at a moderate sparging rate. After sparging, a 100 ml sample was immediately collected from under the foam layer. The results are summarized in Table 1.

                  TABLE 1                                                          ______________________________________                                                                       Second                                                             First Sample                                                                               Sample                                           Original          (3 hr Sparge)                                                                              (1 hr Sparge)                                    Sample            20 min Settling                                                                            (No Settling)                                    ______________________________________                                         NaOH   48.8% (by weight)                                                                             --          --                                           NaCl    1.38% (by weight)                                                                            1.0%        1.12%                                        Na.sub.2 SO.sub.4                                                                     1419 ppm (by weight)                                                                          <60 ppm     858 ppm                                      ______________________________________                                    

The lower NaCl and Na₂ SO₄ values obtained in the first sample versus those obtained in the second sample are believed to result from (1) the additional time of sparging (3 hours v. 1 hour) and (2) the 20 minute settling time in the first sample which allowed suspended solids to collect on the bottom of the beaker.

The purification procedure followed for the second sample with its one hour sparge and no settling time approximates actual commercial NaOH plant operating conditions more closely than does the purification procedure followed for the first sample.

Nos. 3 to 8

The following first stage purifications of approximately 50% aqueous NaOH utilized a 20-gallon drum inside a 55-gallon drum. Caustic, reported in gallons per minute, was fed into the smaller drum about 6 inches from the bottom; air, reported in cubic feet per minute (cfm), was then sparged into this smaller drum. Air feed rate was measured with a rotameter. The larger drum provided overflow facilities and was fitted with a weir to remove foam as it separated. All temperatures were between 25° C. to 31° C. Results are summarized in Table 2.

                  TABLE 2                                                          ______________________________________                                         First                                                                          Stage Feed     Air     Feed Sample                                                                               Product Sample                               No.   (gpm)    (cfm)   NaCl  Na.sub.2 SO.sub.4                                                                     NaCl  Na.sub.2 SO.sub.4                    ______________________________________                                         3     0.55     0.1     1.24  1966   1.11  1385                                 4     0.55     0.1     1.22  1749   1.11  1029                                 5.sup.1                                                                              0.8      0.1     1.19  1866   1.13  1129                                 6.sup.2                                                                              0.55     0.1     1.29  2324   1.08   463                                 7.sup.3                                                                              0.11     0.1     1.29  2324   1.03   612                                 8     Batch    0.067   1.16  1577   1.09  1158                                       5.4                                                                            gallons                                                                  ______________________________________                                          .sup.1 29° C.                                                           .sup.2 Three hour sparge followed by onehalf hour settling;                    .sup.3 Two hour sparge, no settling.                                     

The results summarized in Table 2 demonstrate the importance of even gas flow relatively well-spread out over the entire volume of the impurity-containing aqueous NaOH. In the absence of such controlled gas flow, poor separation efficiencies may result.

Nos. 9 to 73

The following first stage purifications were made in a twenty cubic foot, four cell, Denver Equipment Company pilot aeration-flotation unit, Model No. 5-6.5. Air was the gas employed. Table 3 lists air input in Standard cubic feet per minute (scfm). Aqueous NaOH feed is given in gallons per minute (gpm). The concentration of NaCl and NaOH is in percent by weight and the concentration of Na₂ SO₄ is in parts per million (ppm) by weight. The temperature was between 25° C. to 31° C. in all instances. The numbered sets of data (wherein 9-15 is a set of data, 16 to 19 is another set, etc.) were obtained at one hour intervals after conditions reached equilibrium. Feed concentrations of NaOH varied between about 46% to 50%. Concentrations of NaOH in the product samples closely approximated concentrations of NaOH in the corresponding feed samples.

                  TABLE 3                                                          ______________________________________                                         First                                                                          Stage Feed     Air     Feed Sample                                                                               Product Sample                               No.   (gpm)    (scfm)  NaCl  Na.sub.2 SO.sub.4                                                                     NaCl  Na.sub.2 SO.sub.4                    ______________________________________                                          9    12       Not     1.30  1792   1.09  603                                  10    20.5     Meas-   ↓                                                                             ↓                                                                              1.07  553                                  11    20.5     ured    ↓                                                                             ↓                                                                              1.08  814                                  12    12               ↓                                                                             ↓                                                                              1.09  462                                  13    28               ↓                                                                             ↓                                                                              1.10  676                                  14    6.5              ↓                                                                             ↓                                                                              1.09  220                                  15    20.5             ↓                                                                             ↓                                                                              1.10  561                                  16    12       Not     1.28  2068   1.09  589                                  17    10       Meas-   ↓                                                                             ↓                                                                              1.10  380                                  18    10       ured    ↓                                                                             ↓                                                                              1.10  205                                  19    6.5              ↓                                                                             ↓                                                                              1.14  183                                  20    11.5     2.55    1.28  2051   1.11  620                                  21    11.5     3.83    ↓                                                                             ↓                                                                              1.10  561                                  22    11.5     5.10    ↓                                                                             ↓                                                                              1.09  572                                  23    11.5     6.38    ↓                                                                             ↓                                                                              1.11  557                                  24    11.5     8.30    1.25  1940   1.09  463                                  25    11.5     11.48   1.25  1940   1.10  543                                  26    11.5     5.74    1.23  1487   1.11  537                                  27    11.5     5.74    ↓                                                                             ↓                                                                              1.11  197                                  28    12       5.74    ↓                                                                             ↓                                                                              1.10  240                                  29    12       5.74    ↓                                                                             ↓                                                                              1.10  573                                  30    12       8.55    ↓                                                                             ↓                                                                              1.08  521                                  31    12       8.55    ↓                                                                             ↓                                                                              1.16  545                                  32    16       8.55    ↓                                                                             ↓                                                                              1.08  602                                  33    16       8.55    ↓                                                                             ↓                                                                              --    --                                   34    16       5.74    ↓                                                                             ↓                                                                              1.11  835                                  35    12       2.5     1.42  1687   1.26  781                                  36    12       2.5     ↓                                                                             ↓                                                                              1.21  676                                  37    12       1.3     ↓                                                                             ↓                                                                              1.19  829                                  38    16       1.3     ↓                                                                             ↓                                                                              1.18  890                                  39    5        1.3     ↓                                                                             ↓                                                                              1.45  720                                  40    5        1.3     ↓                                                                             ↓                                                                              1.27  470                                  41    5        2.5     ↓                                                                             ↓                                                                              1.21  409                                  42    20       6.4     1.29  1515   1.09  575                                  43    15       6.4     ↓                                                                             ↓                                                                              --    --                                   44    15       6.4     ↓                                                                             ↓                                                                              --    --                                   45    15       3.8     ↓                                                                             ↓                                                                              --    --                                   46    15       2.6     ↓                                                                             ↓                                                                              --    --                                   47    15       1.3     ↓                                                                             ↓                                                                              1.12  725                                  48    12       2.5     1.28  2015   1.13  627                                  49    12       1.3     ↓                                                                             ↓                                                                              1.10  656                                  50    16       1.3     ↓                                                                             ↓                                                                              1.11  971                                  51    5        1.3     ↓                                                                             ↓                                                                              1.10  589                                  52    12       1.9     ↓                                                                             ↓                                                                              1.11  762                                  53    12       2.5     ↓                                                                             ↓                                                                              1.11  483                                  54    8        6.4     2.89  3973   1.09  578                                  55    8        3.8     ↓                                                                             ↓                                                                              1.10  666                                  56    8        3.8     ↓                                                                             ↓                                                                              1.09  681                                  57    5        3.8     ↓                                                                             ↓                                                                              1.07  424                                  58    12       3.8     1.35  2358   1.09  794                                  59    12       3.8     ↓                                                                             ↓                                                                              1.08  541                                  60    12       3.8     ↓                                                                             ↓                                                                              1.08  581                                  61    12       3.8     ↓                                                                             ↓                                                                              1.06  681                                  62    12       3.8     ↓                                                                             ↓                                                                              1.07  614                                  63    5        2.5     2.56  3961   1.14  517                                  64    5        2.5     ↓                                                                             ↓                                                                              1.09  528                                  65    5        2.5     ↓                                                                             ↓                                                                              1.07  407                                  66    5        2.5     ↓                                                                             ↓                                                                              1.07  532                                  67    5        2.5     ↓                                                                             ↓                                                                              1.09  543                                  68    5        2.5     2.48  3926   1.09  535                                  69    5        2.5     3.04  7660   1.16  619                                  70    5        2.5     ↓                                                                             ↓                                                                              1.09  480                                  71    5        2.5     1.98  2054   1.13  433                                  72    5        2.5     ↓                                                                             ↓                                                                              1.12  598                                  73    5        2.5     ↓                                                                             ↓                                                                              1.11  414                                  ______________________________________                                    

SECOND STAGE (S.S.) SEPARATIONS

The experiments summarized in Table 4, below, were run at 34° to 35° C. with a feed stream consisting of 1.31% NaCl, 1657 ppm Na₂ SO₄ and 49.94% NaOH. NaCl levels in the product were higher at second stage than at first stage temperatures due to greater salt solubility at second stage temperatures. However, unexpectedly, Na₂ SO₄ levels were lower than they were at first stage temperatures. This latter observation led to the hypothesis that, in an important sense, sulfate removal is more dependent on viscosity than on solubility.

                  TABLE 4                                                          ______________________________________                                         Second                                                                         Stage Feed     Air     Feed Sample                                                                               Product Sample                               Letter                                                                               (gpm)    (scfm)  NaCl  Na.sub.2 SO.sub.4                                                                     NaCl  Na.sub.2 SO.sub.4                    ______________________________________                                         A     5        2.5     1.31  1657   1.14  <60                                  B     5        2.5     ↓                                                                             ↓                                                                              1.17  <60                                  C     5        2.5     ↓                                                                             ↓                                                                              1.13   79                                  D     5        2.5     ↓                                                                             ↓                                                                              1.13   175                                 E     5        2.5     ↓                                                                             ↓                                                                              1.16   77                                  F     5        2.5     ↓                                                                             ↓                                                                              1.12   147                                 ______________________________________                                    

Subsequently, experiments were run at successively higher temperatures: 28° C. (F.S. 63 to 68; Table 3), 31° C. (F.S. 69 to 73; Table 3), and 34° C. (S.S. G to K; Table 5 below). The results demonstrated that as temperature was increased, salt removal became less efficient and sulfate removal became more efficient.

From the data of F.S. 63 to 68 and that of S.S. A to F, it is concluded that if product from a first stage such as Nos. 63 to 68 were fed to a second stage such as that summarized in Table 4 (A to F), the final product would have both low NaCl and low Na₂ SO₄.

                  TABLE 5                                                          ______________________________________                                         Second                                                                         Stage Feed     Air     Feed Sample                                                                               Product Sample                               Letter                                                                               (gpm)    (scfm)  NaCl  Na.sub.2 SO.sub.4                                                                     NaCl  Na.sub.2 SO.sub.4                    ______________________________________                                         G     5        2.5     2.83  3800   1.20  581                                  H     5        2.5     ↓                                                                             ↓                                                                              1.18  459                                  I     5        2.5     ↓                                                                             ↓                                                                              1.23  396                                  J     5        2.5     ↓                                                                             ↓                                                                              1.20  450                                  K     5        2.5     2.50  4100   1.19  367                                  ______________________________________                                    

The tests summarized in Table 6 duplicate those summarized in Table 4. They support the conclusion that Na₂ SO₄ removal is more efficient at second stage temperatures of 34° to 35° C. than at first stage temperatures of 25° C. to 31° C.

                  TABLE 6                                                          ______________________________________                                         Second                                                                         Stage Feed     Air     Feed Sample                                                                               Product Sample                               Letter                                                                               (gpm)    (scfm)  NaCl  Na.sub.2 SO.sub.4                                                                     NaCl  Na.sub.2 SO.sub.4                    ______________________________________                                         L     5        2.5     1.23  2027   1.09  7371                                 M     5        2.5     1.22  1729   1.10  <60                                  ______________________________________                                    

EXAMPLES 1 TO 73

Based on the data summarized in First Stage Purification Nos. 1 to 73 together with the Second Stage data (A to M) presented in Tables 4 to 6, it is concluded that subjecting the aqueous NaOH from the first stage flotation separation to a second stage flotation separation in accordance with this invention will result in removal from the aqueous NaOH of substantially all the suspended Na₂ SO₄ impurities remaining after the first stage flotation purification. Substantially all of the suspended NaCl will, of course, have been removed in the first stage. 

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
 1. A process for purifying an aqueous solution containing less than 52 weight percent of NaOH and containing suspended NaCl and Na₂ SO₄ impurities, comprising:(i) bubbling a gas into the impurity-containing NaOH solution at a temperature above 16° C. up to 31° C., to transport suspended impurities to the surface of the NaOH solution and form a surface foam layer on the NaOH solution; (ii) separating the surface foam layer of impurities from the NaOH solution; (iii) bubbling a gas into the NaOH solution from step (ii) at a temperature above 31° up to about 36° C., to transport suspended impurities to the surface of the NaOH solution and form a surface foam layer on the NaOH solution; (iv) separating the surface foam layer of impurities from the aqueous NaOH solution;whereby substantially all of the suspended NaCl impurity is removed by steps (i) and (ii); whereby substantially all of the suspended Na₂ SO₄ impurity remaining after steps (i) and (ii) is removed by steps (iii) and (iv).
 2. A process according to claim 1 wherein the temperature of step (i) is about 25° C. to 29° C. and the temperature of step (iii) is about 32° C. to 34° C.
 3. A process according to claim 2 wherein the aqueous NaOH has a concentration of about 45% to <52%.
 4. A process according to claim 3 wherein the gas is air.
 5. A process according to claim 2 wherein the gas is air or nitrogen.
 6. A process according to claim 5 wherein the gas is air.
 7. A process according to claim 1 wherein the aqueous NaOH has a concentration of about 45% to <52%.
 8. A process according to claim 7 wherein the gas is air.
 9. A process according to claim 1 wherein the gas is air or nitrogen.
 10. A process according to claim 9 wherein the gas is air. 